Process for cleaning harmful gas

ABSTRACT

There is disclosed a process for cleaning a harmful gas which comprises bringing a harmful gaseous halogenide such as chlorine, hydrogen chloride, dichlorosilane, silicon tetrachloride, phosphorus trichloride, chlorine trifluoride, boron trichloride, boron trifluoride, tungsten hexafluoride, silicon tetrafluoride, fluorine, hydrogen fluoride and hydrogen bromide into contact with a cleaning agent comprising zinc oxide, aluminum oxide and an alkali compound to remove the above halogenide. The above process is extremely effective for promptly and efficiently removing the above gaseous halogenide that is contained in the gas discharged from semiconductor manufacturing process or leaked suddenly from a gas bomb in an emergency.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for cleaning a harmful gas.More particularly, it pertains to a process for cleaning ahalogenide-based harmful gas such as dichlorosilane, hydrogen chloride,tungsten hexafluoride and chlorine trifluoride that are used in thesemiconductor manufacturing industry.

There has been a steady rise in the kinds and usage of gaseoushalogenide such as hydrogen chloride, dichlorosilane, chlorinetrifluoride and tungsten hexafluoride in recent years as thesemiconductor industry and the optoelectronics industry continue todevelop. The above-mentioned gases are employed for the formation ofcrystalline silicon, amorphous silicon or silicon oxide films in theproduction industries of silicon semiconductors and chemical compoundsemiconductors and are indispensable substances as etching gases.However, since each of the aforesaid gases exerts a harmful influence onhuman bodies and the environment on account of their high toxicity, theyneed to be removed when contained in a gas used in the semiconductorproduction industry, prior to the discharge in the atmosphere, afterbeing used in the industry.

In addition, hydrolyzable or comparatively less toxic gases such ascarbon tetrafluoride, perfluoropropane and sulfur hexafluoride are alsoemployed for the dry etching of silicon films and silicon oxide films ina semiconductor production process, but in the gas discharged throughthe etching process there is formed a harmful component such as silicontetrafluoride and fluorine by the reaction between the above-mentionedgases and the film component or by the decomposition of the gases. Suchbeing the case, sufficient care should be taken in discharging a gasfrom the process.

The aforementioned gases are usually available on the market in thestate that the gas is filled in a 0.1 to 47 liters bomb as a pure gas ora diluted gas with nitrogen or the like as exemplified hereunder.

Examples of commercially available gases filled in bombs are as follows:

    ______________________________________                                        Gas                                                                           (chemical                                                                            Bomb capacity                                                                             Concentration                                                                             Weight of filled gas                           formula)                                                                             (liter)     (%)         (kg)                                           ______________________________________                                        HCl    47          100         25                                             SiH.sub.2 Cl.sub.2                                                                   10          100         10                                             ______________________________________                                    

In order to prevent the atmospheric air from being directly pollutedwith a gas in case of its leakage from a bomb, the bomb is used in thestate that it is connected to a gas supply piping connected to asemiconductor process while it is housed in a bomb accomodation vessel,so called a bomb box that is connected to a ventilation duct. However,even if a bomb is housed in a bomb box, complete prevention of suchdangerous accident that is not secured when the bomb is emptied withinonly about 5 to 10 minutes by a sudden leakage of the gas therefrom.Under such circumstances, there is eagerly desired a complete andthorough countermeasure capable of sufficiently coping with suchaccident.

2. Description of the Related Art

As the means for removing gaseous halogenide such as hydrogen chloride,dichlorosilane and boron trifluoride that is contained is a process gas,there have heretofore been used two processes, one being a wet processin which the gaseous halogenide is absorbed and decomposed in a scrubberand the other being a dry process wherein the harmful gas is cleaned bybeing passed through a packed column which is packed with a cleaningagent comprising an alkaline component impregnated into a porous carriersuch as activated carbon, a cleaning agent comprising soda lime as theeffective ingredient or a like agent.

However, the above-mentioned wet process generally suffers thedisadvantages that it finds difficulty in post-treatment and furtherthat it requires a considerable expense in the maintenance of theequipment used therefor.

On the other hand, the aforestated dry process involves such problemsthat there are not necessarily obtained a sufficient removal rate and asufficient removal capacity of the harmful gas with a cleaning agentcomprising sodium hydroxide, potassium hydroxide or the like impregnatedinto a porous carrier such as activated carbon or a cleaning agentcomprising soda lime and that the dry process fails to promptly copewith a highly concentrated harmful gas and an emergency of sudden orviolent leakage of such gas. In addition, a cleaning agent supported onactivated carbon sometimes forms inflammable substances with anextremely reactive gas such as fluorine, involving the hazard of fire.On the contrary, soda lime is liable to cause plugging in a cleaningcolumn due to its deliquescent property in combination with the moisturecontained in the gas to be treated and to form remarkably deliquescentcalcium chloride when used for passing gaseous chloride such as hydrogenchloride and dichlorosilane. As the result, it becomes a problem thatthe use of soda lime limits the kinds of gases suitable to be treated.

Such being the case, there has long been desired the materilization of aprocess for cleaning a harmful gas, especially that discharged fromsemiconductor manufacturing industry. There has also been desired aprocess being high in treatment rate and treatment capacity for harmfulgases, excellent in removal performance, not only in an emergency when arelatively less concentrated, but a large amount of a harmful gas isleaked from a gas bomb because of its abnormality, but also in theordinary case where a concentrated harmful gas is discharged from asemiconductor manufacturing industry, and free from the possibility ofcausing a hazard fire in the case of gas cleaning or the plugging in acleaning column due to deliquesence of a cleaning agent.

In view of the aforestated situation, intensive research andinvestigation were concentrated by the present inventors on the solutionof the above-described problems. As a result it has been found by themthat the use of a cleaning agent comprising zinc oxide, aluminum oxideand an alkali compound can remove a harmful gas in various states withextremely high efficiency and utmost safety. The present invention hasbeen accomplished on the basis of the above-mentioned finding andinformation.

SUMMARY OF THE INVENTION

Specifically, the present invention provides a process for cleaning aharmful gas which comprises bringing a harmful gas containing a gaseoushalogenide as the harmful component into contact with a cleaning agentcomprising zinc oxide, aluminum oxide and an alkali compound to removesaid harmful component.

The process according to the present invention is capable of efficientlyremoving a halogenide-based harmful gas such as chlorine, hydrogenchloride, dichlorosilane, silicon tetrachloride, phosphorus trichloride,chlorine trifluoride, boron trichloride, boron trifluoride, tungstenhexafluoride, silicon tetrafluoride, fluorine, hydrogen fluoride orhydrogen bromide contained in air, nitrogen, hydrogen or the like. Inparticular as mentioned hereinbefore, an excellent effect is exerted bythe present process, not only on the prompt cleaning of a lessconcentrated and a large amount of gas (usually air) polluted by aharmful gas that suddenly leaks from a gas bomb but also on the cleaningof a generally concentrated harmful gas at a constant flow rate that isdischarged from a semiconductor processing unit, which harmful gas hasheretofore been difficult to clean by any of the conventional processes.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic process flow diagram showing the process forcleaning a harmful gas.

Symbols

1: Cleaning column

2: Gas bomb

3: Bomb box

4: Blower

5: Ventilating duct

DESCRIPTION OF PREFERRED EMBODIMENTS

The cleaning agent to be employed in the process according to thepresent invention comprises zinc oxide, aluminum oxide and an alkalicompound as the principal components.

The zinc oxide can be selected from suitable ones available on themarket or from the precursors thereof that are convertible into zincoxide by means of calcination or the like, said precursors beingexemplified by zinc carbonate, basic zinc carbonate, zinc hydroxide andzinc salt from organic acid.

As the aluminum oxide, there is usually used hydrated alumina, andsuitable examples include alumina sol available on the market andconcentrated alumina produced by powdering the alumina sol.

Examples of the alkali compound include a hydroxide, a carbonate and anacetate each of an alkali metal such as lithium, sodium and potassium;an alkaline earth metal such as magnesium and calcium; or ammonium, ofwhich are preferable potassium carbonate, potassium hydrogencarbonate,potassium hydroxide, sodium carbonate, sodium hydrogencarbonate, sodiumhydroxide, ammonium hydroxide and a mixture of at least two thereof.

The ratio of the aluminum oxide to the zinc oxide is usually 0.02 to0.60, preferably 0.05 to 0.60 expressed in terms of aluminum atom andzinc atom (the number of aluminum atoms per one zinc atom), and theratio of the alkali compound to the zinc oxide is usually 0.02 to 0.70,preferably 0.05 to 0.50 expressed in terms of alkali metal or ammoniumgroup and zinc atom. In addition, there can be contained a small amountof a metallic component other than zinc such as copper, chromium, iron,nickel or cobalt.

Various methods are available for the preparation of the cleaning agentand exemplified by a method in which water is added to a mixture of zincoxide or a precursor thereof, alumina sol and an alkali compound withkneading or to a mixture of a precursor thereof and alumina sol to forma cake, and the mixture or cake thus obtained is dried to be used as thecleaning agent as such or made into molding.

In the case where the cleaning agent is packed in a cleaning column tobe used for cleaning the harmful gas, it is preferably molded intopellet or the like.

A wide variety of molding methods are available and exemplified by amethod wherein the above-obtained mixture or cake is extrusion-moldedinto a pellet followed by drying the pellet; a method wherein the cakeis dried, ground and incorporated with a lubricant such as graphite, andthe mixture is molded into a tablet; and a method wherein the cake isgranulated by the use of a granulator or the like.

In general, pelletization by means of extrusion molding is convenientlyapplied in view of the workability and facility in the selection of theshape and size of the molding, and the pellet is desirably rounded atthe ends thereof by the use of a rounding machine or the like.

The shape and size of the molding are not specifically limited but aretypified by a sphere of 0.5 to 10 mm in diameter, column of 0.5 to 10 mmin diameter and 2 to 20 mm in height, cylinder, etc. There is also usedan irregular shape having a screen opening in the range of 0.84 to 5.66mm, approx. The molding has a density in the range of usually 0.5 to 3.0g/ml, preferably 0.7 to 2.5 g/ml. By the term "density" as mentionedherein is meant the weight of a molding (granule) divided by thegeometrical volume thereof.

The bulk density of the moldings when they are packed in a cleaningcolumn is usually 0.4 to 2.0 g/ml, preferably 0.5 to 1.5 g/ml.

The cleaning agent to be used in the present invention can be used inthe form of a mobile bed or a fluidized bed in addition to a fixed bed.

Under ordinary circumstances, the cleaning agent is packed in a cleaningcolumn, and the gas containing a gaseous halogenide is passedtherethrough and brought into contact with the cleaning agent so thatthe gaseous halogenide as the harmful component is removed.

There is no limitation to the concentration and flow velocity of thegaseous halogenide contained in the gas to be treated to which isapplied the cleaning process of the present invention, but in generalthe flow velocity is desirably lowered with increasing concentration.The eliminable concentration of the harmful gas is usually 1% or lower,but can be increased in the case of low flow rate of the gas containinga gaseous halogenide.

The cleaning column is designed in accordance with the concentration ofharmful gas, the amount of the gas to be treated, etc. The space linearvelocity (LV) is preferably designed at 0.3 to 1.5 m/sec for arelatively low concentration of harmful gas such as 0.1% or lower, 0.05to 0.3 m/sec for a concentration of 0.1 to 1% approx and 0.05 m/sec orlower for a high concentration such as 1% or higher. The general designcriteria specify a LV value of 0.3 to 1.5 m/sec in the case where, forexample, the harmful gas is suddenly leaked from a gas bomb and dilutedwith a large amount of a harmless gas such as air, and a LV value of0.05 to 0.3 m/sec in the case where, for example, the concentratedharmful gas is discharged from the process in a steady state.

The contact temperature between the cleaning agent and the gas to betreated is ordinarily 0° to 90° C., preferably ordinary temperature (10°to 50° C.) without the need for heating or cooling in particular. Thecontact temperature after the initiation of startup sometimes rises tosome extent on account of reaction heat depending upon the concentrationof the harmful gas, but there is no possibility of causing fire sinceany inflammable substance such as activated carbon is not used in thecleaning process.

The pressure during the contact therebetween is usually atmosphericpressure, but may be reduced pressure or raised pressure such as 1kg/cm² G.

The humidity of the harmful gas to which is applied the cleaning processof the present invention is not specifically limited, but may be in thedry or wet state of the gas insofar as moisture condensation is notcaused. The above-mentioned conditions are free from a fear of causingdeliquescence of the cleaning agent or deterioration of performancethereof.

In addition, carbon dioxide gas in the air does not exert adverseinfluence upon the cleaning agent.

In the case when a sudden leakage of harmful gas is assumed in a bombbox, the process of the present invention is prepared in such a statethat, as shown in the flow diagram of FIG. 1, a cleaning column 1 isinterposed between a ventilating duct 5 connected to a bomb box 3 thathouses a gas bomb 2 and a blower 4 used for continuouslysuction-ventilating the air in the bomb box 3.

The above-mentioned system is usually equipped with a blower having acapacity sufficient for diluting the harmful gas as low as aconcentration of 1% or lower by mixing it with air even if a suddenleakage of the harmful gas should take place. Specifically, there isgenerally installed a blower having a ventilation capacity of 5 to 200m³ /min, leading to the assumption that the content of the harmful gasin the air becomes about 50 to 1000 ppm when a leakage takes place insuch an extent that a commercially available gas bomb as mentioned inthe aforestated table is emptied within 5 to 10 minutes.

The height of packing of the cleaning agent in the cleaning columnvaries depending upon the flow rate of the gas to be cleaned, theconcentration of the harmful gas, etc. and thus can not be specifiedunequivocally, but is usually set to 50 to 500 mm from the practicalviewpoint. The inside diameter of the cleaning column is designed so asto attain about 0.3 to 1.5 m/sec space linear velocity (LV) for the gasthat is passed through the column. The above-mentioned height and insidediameter are specifically determined taking into consideration thepressure loss through the packed layer, the contact efficiency of thegas with the packing, the concentration of the harmful gas, etc.

The process for cleaning a harmful gas according to the presentinvention can efficiently and extremely promptly remove a relativelyconcentrated and a large amount of halogenide-based harmful gases suchas hydrogen chloride, dichlorosilane, tungsten hexafluoride and chlorinetrifluoride. Accordingly, the above process exerts an excellent effecton the cleaning, not only of a concentrated harmful gas that isdischarged from a semiconductor manufacturing process, etc., but also ofa large amount of harmful gas that is leaked suddenly from a gas bomb inan emergency.

In the following, the present invention will be described in more detailwith reference to the non-limitative examples.

EXAMPLES 1 to 13

Basic zinc carbonate in an amount of 500 g, 52.4 g of concentratedalumina (produced by Catalysts & Chemicals Industries Co., Ltd. underthe tradename "Cataloid AP"), that is, Al/Zn atomic ratio being 0.16 and30.2 g of potassium carbonate anhydride, that is, the K/Zn atomic ratiobeing 0.10 were placed in a small-sized kneader, mixed for 3 minutes andkneaded with 280 g of water added thereto for one hour. The resultantcake was extruded through a nozzle plate with 1.9 mm diameter nozzleattached to a small-sized extruder. The pellet thus obtained was roundedwith a rounding machine and dried at 110° C. for 2 hours, and therounded dried pellet was placed in a muffle furnace and calcined at 350°C. for one hour to obtain a cleaning agent having 1.14 g/ml density.

By the use of the above-obtained cleaning agent, cleaning experimentswere carried out for relatively less concentrated but large amounts ofharmful gases based on the assumption that the harmful gases suddenlyleaked from a bomb. In this experiment, 28.4 ml of the cleaning agentwas packed in a cleaning column made of quartz glass having 19 mm insidediameter and 2000 mm length at a bulk density of 0.98 g/ml. Then,various gaseous halogenides each contained in nitrogen in 1000 ppm wereeach passed through the column at a total flow rate of 10.22 L/min, thatis, space linear velocity of LV=60 cm/sec at 20° C. under atmosphericpressure. The break through point in the experiment was detected bysampling the gas at the outlet of the column and analyzing the samplethus collected by means of a gas-detecting tube for chloride or fluoride(produced by Gastech Coporation). The results are given in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                    Concentration                                                                         Space linear                                                                         Effective treatment time                                       of harmful gas                                                                        velocity LV                                                                          (Break through time)                           Example                                                                            Kind of harmful gas                                                                      (ppm)   (cm/sec)                                                                             (min)                                          __________________________________________________________________________    1    Hydrogen chloride                                                                        1000    60     112                                            2    Dichlorosilane                                                                           1000    60     45                                             3    Boron trichloride                                                                        1000    60     20                                             4    Silicon tetrachloride                                                                    1000    60     26                                             5    Chlorine   1000    60     42                                             6    Silicon tetrafluoride                                                                    1000    60     23                                             7    Phosphorus trichloride                                                                   1000    60     23                                             8    Fluorine   1000    60     97                                             9    Chlorine trifluoride                                                                     1000    60     49                                             10   Tungsten hexafluoride                                                                    1000    60     23                                             11   Hydrogen bromide                                                                         1000    60     83                                             12   Hydrogen fluoride                                                                        1000    60     40                                             13   Boron trifluoride                                                                        1000    60     25                                             __________________________________________________________________________

EXAMPLES 14 to 26

Cleaning experiments were carried out for concentrated harmful gaseseach at a constant flow rate based on the assumption that the harmfulgases were discharged from a semiconductor process.

Various gaseous halogenides each contained in nitrogen in 1 vol % wereeach passed through the same cleaning column that was used in Examples 1to 13 at a total flow rate of 850 ml/min, that is, a space linearvelocity of LV=1 cm/sec at 20° C. under atmospheric pressure. The breakthrough point in the experiment was detected and the break through timewas measured in the same manner as in Examples 1 to 13. The results aregiven in Table 2.

                                      TABLE 2                                     __________________________________________________________________________                    Concentration                                                                         Space linear                                                                         Effective treatment time                                       of harmful gas                                                                        velocity LV                                                                          (Break through time)                           Example                                                                            Kind of harmful gas                                                                      (%)     (cm/sec)                                                                             (min)                                          __________________________________________________________________________    14   Hydrogen chloride                                                                        1       5      241                                            15   Dichlorosilane                                                                           1       5      80                                             16   Boron trichloride                                                                        1       5      52                                             17   Silicon tetrachloride                                                                    1       5      55                                             18   Chlorine   1       5      25                                             19   Silicon tetrafluoride                                                                    1       5      33                                             20   Phosphorus trichloride                                                                   1       5      52                                             21   Fluorine   1       5      156                                            22   Chlorine trifluoride                                                                     1       5      37                                             23   Tungsten hexafluoride                                                                    1       5      64                                             24   Hydrogen bromide                                                                         1       5      196                                            25   Hydrogen fluoride                                                                        1       5      89                                             26   Boron trifluoride                                                                        1       5      80                                             __________________________________________________________________________

Comparative Examples 1 to 8

Two types of cleaning agents were prepared, one by impregnatingactivated carbon as the carrier with sodium hydroxide in an amount of40% by weight and drying the agent at 120° C. to a moisture content of10% by weight to be used in comparative Examples 1 to 4, and the otherby impregnating activated carbon as the carrier with potassium hydroxidein an amount of 50% by weight and drying the agent in the same manner tobe used in Comparative Examples 5 to 8. By the use of the above-preparedcleaning agents, cleaning experiments were conducted for the harmfulgases containing hydrogen chloride and dichlorosilane, respectively inthe same manner as in the above examples. The results are given in Table3.

                                      TABLE 3                                     __________________________________________________________________________                                    Effective                                                              Space linear                                                                         treatment time                                Comparative      Concentration                                                                         velocity LV                                                                          (Break through                                Example                                                                              Kind of harmful gas                                                                     of harmful gas                                                                        (cm/sec)                                                                             time) (min)                                   __________________________________________________________________________    1      Hydrogen chloride                                                                       1%       5                                                   2      Dichlorosilane                                                                          1%       5                                                   3      Hydrogen chloride                                                                       500 ppm 60     12                                            4      Dichlorosilane                                                                          500 ppm 60      8                                            5      Hydrogen chloride                                                                       1%       5                                                   6      Dichlorosilane                                                                          1%       5                                                   7      Hydrogen chloride                                                                       500 ppm 60     18                                            8      Dichlorosilane                                                                          500 ppm 60     12                                            __________________________________________________________________________

What is claimed is:
 1. A process for cleaning a harmful gas whichcomprises a step of: contacting a harmful gas containing a gaseoushalogenide as a harmful component with a cleaning agent comprising (i)zinc oxide, (ii) aluminum oxide and (iii) at least one compound selectedfrom the group consisting of potassium carbonate, potassiumhydrogencarbonate, potassium hydroxide, sodium carbonate, sodiumhydrogencarbonate, sodium hydroxide and ammonium hydroxide, to removesaid harmful gas; the gaseous halogenide being at least one memberselected from the group consisting of chlorine, hydrogen chloride,dichlorosilane, silicon tetrachloride, phosphorus trichloride, chlorinetrifluoride, boron trichloride, boron trifluoride, tungstenhexafluoride, silicon tetrafluoride, fluorine, hydrogen fluoride andhydrogen bromide; a ratio of the aluminum oxide to the zinc oxide being0.05 to 0.60 expressed in terms of a number of aluminum atoms per onezinc atom and a ratio of the compound (iii) to the zinc oxide being 0.05to 0.50 expressed in terms of a number of potassium atoms, or sodiumatoms or ammonium groups per one zinc oxide; the gaseous halogenidehaving a concentration of 50 to 1,000 ppm by volume; and the aluminumoxide being hydrated alumina.
 2. The process according to claim 1wherein the harmful gas is discharged from a semiconductor process. 3.The process according to claim 2 wherein the harmful gas is leaked froma bomb containing said gaseous halogenide and air which dilutes saidgaseous halogenide.
 4. The process according to claim 1 wherein theprocess is carried out at a space linear velocity of 0.3 to 1.5m/second.
 5. The process according to claim 1 wherein the process iscarried out at a space linear velocity of 0.05 to 0.3 m/second.
 6. Theprocess according to claim 1 wherein the process is carried out at atemperature of 0° to 90° C.
 7. The process according to claim 1 whereinthe process is carried out at a temperature of 10° to 50° C.
 8. Aprocess for cleaning a harmful gas which comprises a step of a:contacting a harmful gas containing a gaseous halogenide as a harmfulcomponent with a cleaning agent comprising (i) zinc oxide, (ii) aluminumoxide and (iii) at least one compound selected from the group consistingof potassium carbonate, potassium hydrogencarbonate, potassiumhydroxide, sodium carbonate, sodium hydrogencarbonate, sodium hydroxideand ammonium hydroxide, to remove said harmful gas; the gaseoushalogenide being at least one member selected from the group consistingof chlorine, hydrogen chloride, dichlorosilane, silicon tetrachloride,phosphorus trichloride, chlorine trifluoride, boron trichloride, borontrifluoride, tungsten hexafluoride, silicon tetrafluoride, fluorine,hydrogen fluoride and hydrogen bromide; a ratio of the aluminum oxide tothe zinc oxide being 0.02 to 0.60 expressed in terms of a number ofaluminum atoms per one zinc atom and a ratio of the compound (iii) tothe zinc oxide is 0.02 to 0.70 expressed in terms of a number ofpotassium atoms, sodium atoms or ammonium groups per one zinc oxide; thegaseous halogenide having a concentration of 50 to 1,000 ppm by volume;and the aluminum oxide being hydrated alumina.